These days is a great deal of malicious software that affects not only a user's computer at home or in the office, but may also affect any other public computer or temporary computer that a user might use when on the road or away from the office. Especially when a user is using a public computer or a computer other than their own, there is no guarantee that any required antivirus software has been installed or is in use. It is entirely possible that malicious software might modify the operating system or settings of this other computer of the user without the user noticing. For example, even though the user may have backed up his operating system settings on his own office computer, he is unable to take those settings with him if he travels and uses another computer. This malicious software may even modify the operating system of the user's own computer if antivirus software is not being used.
Examples of malicious software that might affect an operating system or its settings include malware that runs immediately upon system startup, malware that modifies an operating system causing it to be prone to a pharming attack, malware that performs DNS spoofing, malware that hijacks an Internet browser, etc. For example, malware might destroy or modify operating system settings, or the malware might destroy a restore point for the operating system that the user had previously created in order to remedy a malware attack.
It is desirable for user to be able to recover from such a malware attack that affects an operating system, its settings, or the state of a computer.
Furthermore, it is usually difficult if not impossible for a user to determine that a public computer he or she is using (such as a computer at a library, Internet café, hotel business center, etc.) is free for many malware. Either there is no antivirus software present on the computer or the user is unable or unwilling to run such software before beginning work. In particular, in the past a user of a public computer was required to download special antivirus software or perhaps do an online scan in order to ensure that no malicious software such as key logger software or Trojan horse software was present on the public computer. Absent these procedures, a user had to be extremely careful about typing in sensitive information. Another difficulty of using a public computer is that the user may require certain software for his or her activities that is not present on the public computer.
It is therefore desirable for a user to be able to use a public computer without having to worry about malicious software or the absence of needed software.